In this work, zinc oxide (ZnO) nanoballs were directly deposited on porous silicon (PS) substrates using RF magnetron sputtering method. The PS substrates were synthesised at room temperature via metal assisted chemical etching technique. The structural, surface morphological and cross-sectional properties of as prepared ZnO nanoballs were investigated by x-ray diffraction, Raman spectroscopy and field emission scanning electron microscopy techniques, respectively. The interaction of nitrogen dioxide (NO2) gas molecules with sensing layer and all sensing properties under lower detection limit (2 ppm–100 ppm) were studied in detail. The proposed ZnO nanoballs sensor chip exhibits high sensing response (29.2%) with fast response/recovery times (70 sec/96 sec) to 20 ppm NO2 in dry air at 250°C. Thus, the findings recommend the viability of ZnO nanoballs sensor as highly sensitive and selective NO2 gas sensor at moderate operating temperature regime.